Triggerable vacuum discharge devices with a gas producing trigger electrode



y 1967 J. M. LAFFERTY 3,331, 81

VACUUM DISCHARGE DEVICES WITH A GAS PRODUCING TRIGGER ELECTRODE FiledDec. 11, 1964 Inventor:

e M. caffert Mf A 3230 r rvey,

Unitcd States Patent 3,331,981 TRIGGERABLE VACUUM DISCHARGE DE- VICESWITH A GAS PRODUCING TRIG- GER ELECTRODE James M. Latferty, Schenectady,N.Y., assignor to General Electric Company, a corporation of New YorkFiled Dec. 11, 1964, Ser. No. 417,562 4 Claims. (Cl. 313-178) ABSTRACTOF THE DISCLOSURE Discloses a triggerable vacuum gap device containing atrigger assembly having a gas storage trigger member loaded with activegas which is evolved and ionized to cause breakdown of a primary gap andmethod of fabrication and evacuation thereof. Device also contains aremote active gas storage reservoir located remotely from heatingeffects of primary arc and means to selectively heat the reservoir at apredetermined time to facilitate replenishment of the trigger assembly.As the final step in fabricating the device, the interior thereof isflushed with the active gas at a predetermined pressure and the deviceheated and sealed. Upon cooling, the active gas at the predeterminedpressure is completely absorbed by the trigger assembly and the gasreservoir to establish a hard vacuum within the device.

This invention is a continu-ation-in-part of my copending applicationSer. No. 357,089, filed Apr. 3, 1964, now abandoned. The presentinvention relates to electric discharge devices adapted to switch highvoltages and currents by triggering electric breakdown between a pair ofelectrodes separated by a gap in vacuo.

In my US Patent 3,087,092, issued Apr. 23, 1963, entitled G-asGenerating Switching Tube, there is disclosed a triggerable vacuumdischarge device which comprises a pair of primary discharge electrodes,fabricated from gas-free metal, separated by a primary gap and disposedin an envelope evacuated to a pressure of millimeters (mm.) of mercuryor less. A trigger assembly, composed of gas-charged metal, as forexample titanium and having a gap across which a discharge is easilystarted, is provided to release and ionize gas, as for example hydrogen,from the metal thereof and direct the electron-ion plasma formed therebyinto the gap between the primary dis charge electrodes to cause theprimary gap to be broken down in the electric interaction between theinjected plasma and the presence of a high voltage across the primaryelectrodes. When the primary discharge is terminated, the primary gapclears quickly due to diffusion of the electrode material (the ions andelectrons of which are the arc-sustaining particles) and due toabsorption of the triggering gas by the trigger metal and by evaporatedelectrode material. Due to these phenomena, the high dielectric strengthof the original, evacuated gap is re-established rapidly. Until thetrigger discharge is initiated again, the primary gap withstands veryhigh voltages without breakdown.

In the conventional fabrication process for this type device, the deviceenvelope is evacuated, after construction, by baking out at 400 C. orhigher, depending on whether hard glass or ceramic construction is used.After bakeout, the trigger electrode is outgassed at a temperature of1000 C. by passing current through a heater winding. After outgassing,hydrogen is let into the system and the trigger is allowed to coolslowly. Hydrogen is taken up by the titanium and the trigger is loaded.The device is then baked out again at a moderate temperature of, forexample 200 C., which is high enough to remove a considerable amount ofadsorbed gas from the electrodes and device walls, but not high enoughto remove appreci- ICC able hydrogen from the trigger electrode. Aftercooling and low current sparking with high voltage to remove surfaceimpurities from the electrode surfaces the envelope is sealed. Thepressure within the device is 10* mm. of Hg or less, and is maintainedat this value by its own getter until operation.

A method of evacuation has been proposed, in US. Patent No.2,934,392-DeSantis, et a1., issued Apr. 26, 1960, which comprises usingtitanium, or other absorbent metal, to absorb an active gas in theatmosphere of a vacuum tube so as to substantially evacuate the tube.This method has required the provision of large amounts of elementaltitanium in order to accomplish evacuation of moderate or small volumes.While this is not a disadvantage in the fabrication of vacuum tubes andlike devices it becomes uneconomical when practiced upon high current orhigh voltage switching devices.

The present invention therefore is directed to overcoming variousdifficulties associated with the fabrication of triggered vacuumdischarge devices, and to an improvement in such devices.

Accordingly, it is an object of the present invention to provide animproved method of evacuating triggered vacuum gap switching devices.

Another object of the present invention is the provision of an improvedmethod of manufacture of triggerable vacuum discharge devices whichincludes simultaneous evacuation and loading thereof.

Another object of the present invention is the provision of an improvedtriggerable vacuum discharge device, the trigger of which may bereloaded without violating the vacuum in the device.

Yet another object of the present invention is the provision of a methodof reloading the trigger of a triggerable vacuum discharge devicewithout the necessity of violating the vacuum therein.

Briefly, in accord with one aspect of the present invention, I provide amethod of evacuating a triggerable vacuum discharge device adapted tohave a discharge triggered by the release and ionization of an activegas therein, which method comprises the steps of providing a quantity ofactive-gas-charged material in the device, heating the device prior tohermetic sealing thereof to an elevated temperature in an atmosphereconsisting essentially of the active gas, sealing the device andpermitting the device to cool so that the metal absorbs the active gas,thus producing the requisite vacuum and simultaneously loading thetrigger.

In accord with another aspect of the present invention, I provide as anadditional element a further'quantity of active gas charged material,having the active gas incorporated therein, located in the device in anarea which remains relatively cool during operation. After the devicehas been fabricated by the above described method and after a period ofuse in which the quantity of active gas available in the trigger hasbeen depleted, the trigger may be reloaded by heating at least thatportion of the device containing the reservoir to the elevatedtemperature and permitting it to cool.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention, itself, together withfurther objects and advantages thereof may best be understood byreference to the following description taken in connection with theappended drawing which represents a device fabricated in accordance withthe present invention.

The device illustrated in the figure comprises a gasimperviousinsulating envelope 1 which is composed of a flanged disc end wallassembly 2, a cylindrical sidewall member 3 and an end closure member 4.A pair of main gap electrodes 5 and 6 are supported in spaced-apartrelation within envelope 1 to define a primary gap 7. Electrode 5comprises a cylindrical member having an axial aperture therein. Theaperture is tapered outwardly at the exterior portion thereof to providea bore in the end of electrode 6 having an interior cylindrical portion8 andan exterior conical portion 9. Trigger assembly 10 is mountedwithin the aperture in electrode 6.

Electrode 5 is supported Within-the envelope 1 by end wall assembly 2while electrode 7 is supported from end closure member 4 by means ofelectrodesupport rod 11, which is hermetically sealed to the closuremember 4 by welding, brazing or other suitable techniques. Electrode 6may be of any suitable size and configuration so as to properly maintaina discharge with electrode 5.

A metallic shield12, having a generally cylindrical shape with aferruled open end 13to prevent arcing, is suspended from end wallassembly 2 and extends well past the gap between electrodes 5,and 6.Shield 12 is utilized to preclude metal sputtered or evaporated fromelectrodes Sand 6 from completely coating the inner surface ofcylindrical sidewall member 3 of envelope 1 and thus destroying theinsulating characteristics thereof.

Trigger assembly 10 comprises a cylindrical ceramic member 14 coatedwith a thin layer 15 of an electrically conductive gas charged material,such as a hydride of titanium, hafnium, zirconium or thorium which hasincorporated therein an active gas such as hydrogen. After layer 15 hasbeen formed, a groove 16 is scored around the circumference of thecylindrical member so as' to remove the material therefrom and exposethe insulating ceramic. The position of groove 16 is chosen so that,when trigger assembly 10 is positioned within electrode 5, the junctionbetweenthe cylindrical bore 8 and the conical bore 9 is slightly belowthe lower edge of groove 16. A metallic cap 17 is suitably affixed tothe inner end of trigger assembly 10 so as to be in good electricalcontact with layer 15. A wire 18, soldered or otherwise affixed to cap17, extends outwardly through the ceramic member 14 to provide a meansfor applying a trigger potential thereto.

An hermetic seal is completed over the aperture in end wall assembly 2by means of dished member 19,

cylindrical ceramic member 20,.and metallic disc 21. Hermetic seals areformed between each of these members and between disc 21 and wire 18.End wall assembly 2 is provided with a flange 22; end closure member 4is provided with a fiange23, the flanges being adapted to form hermeticseals 24 with ceramic sidewall member 3 during the process of evacuatingthe device, and electrode support, member 11 is hermetically sealed toclosure member 4.

Envelope member 3 and ceramic member 14 are fabricated from agas-impervious, non-conducting material which maybe hermetically sealedto a metal member.

More specifically, it is important that these members, particularlyenvelope member 3, be impervious to helium, since long-term heliumdiffusion through some envelope materials, as for example glass, candestroy the high vacuum necessary for these type devices. Generally, anygas-impervious ceramic may be utilized such as COORS V200 or AmericanLava T164. Alternatively, aluminum oxide or forsterite ceramic bodiesmay be used. It is to be understood, however, that although the specificmaterials have been enumerated, any gas-impervious ceramic or glasswhichmay be hermetically sealed to metal members may also be utilized.

Electrodes 5 and 6 are fabricated from copper that is substantially freeof all gaseous impurities or impurity which, upon decomposition, mayproduce gases. This copper is such that it meets a standard test, thecriterion of which is such that when placed in a vacuumized testchamber, a few litres in volume, and subsequently deeply eroded byrepetitive arcing, as for example, by avoltage of commercial power andcurrent of 100 amperes or more, the pressure level in the container, afew cycles after arcing, does not rise substantially from its initialvalue, in the absence of getters and pumps, even when the initial valueis 10 mm.- of mercury or lower. Analytically this requirement maybestated by the relationship that the contact material must contain lessthan 10 atomic parts of all gases and gas-forming impurities.

The remaining -metallic elements within the envelope such as electrodesupport member 11 need not meet this stringent requirement since theyare not brought into contact with electric arc and therefore are notpotential sources of vacuum-spoiling gases. They should nevertheless beof metal which is completely free of oxygen since, in fabrication, thedevice is subjected to hydrogen at elevated temperatures and it isundesirable to have oxygen as an impurity in any material exposed athigh temperatures to a hydrogen atmosphere.

In fabricating devices such as illustrated, the individual constituentsare prepared and assembled in any convenient fashion, for example, asdescribed in my aforementioned U.S. Patent 3,087,092.

A particular feature of the present invention lies in the material oflayer 15 asprovided prior to evacuation and in the method of evacuatingthe device and loading the trigger. Specifically, it has been foundthat, while the provision of an elemental metal as the material of layer15 is effective in absorbing limited quantities of the active gas inaccord with the aforementioned US. Patent No. 2,934,392, a substantialincrease in the quantity of active gas absorbed thereby'can be achievedif the layer is in the form of the gas-charged metal rather than theelemental metal prior to final 'bakeout and loading. This is due in partto the removal of surface oxides from the elemental metal which areremoved by the prior absorption andremoval of active gas during bakeoutand also in part to the opening of fissures or minute defects in themetal by such prior cycling through which additional active gas canenter and be absorbed in an absorption process. In other words, thequantity of hydrogenabsorbed by a given amount of titanium increases, upto a maximum, as the number of times the titanium-is cycled or if theresulting titanium hydride is heated and cooled in the presence ofhydrogen is increased.

Accordingly, prior to the final procedure of evacuation and loading,layer 15 is pretreated by charging at least once, and preferably severaltimes with hydrogen. This cycling may conveniently 'be done beforedeposition.

A further advantage of utilizinga precharged getter, as for exampletitanium hydride (Til-I over an elemental metal, as for exampletitanium, and requiring the charging thereof during processing, lies inthe relative ease of charging. Thus, if elemental titanium were used andhydrogen were introduced by heating the titanium, surface impurities onthe titanium tend to retard absorption of boundin the TiH is extremelypure. When the entire device is enclosed in an atmosphere of hydrogenand heated to a temperature suflicient to form the seals which form theevacuated envelope, the hydride of the trigger electrode partiallydecomposes, releasing extremely pure hydrogen which flushes theenvelope. interior of the hydrogen atmosphere so that when the envelopeis sealed only, extremely pure hydrogen is present. This precludesnon-absorbable impurities from entering the envelope and limiting thevacuum characteristics of the device. As the sealed device iscooled, thepure hydrogen is re-absorbed by the trigger electrode.

In accord with this invention the hydride does not re-absorb allthehydrogen it held initially. Actualtests have shown the hydride alloy,after formation of seals to be approximately TiH in the case of titaniumhydride. This quantity of hydrogen is quite sufficient to produce ahydrogen ion-electron plasma to break down the main gap upon pulsing.Additionally, it leaves the hydride with the ability to absorb morehydrogen at room or quiescent temperature, should hydrogen be evolvedfrom other parts of the device during arcing.

The final process of evacuation and loading then comprises the followingsteps: with the device completed except for the formation of seals 24,the device is placed in a furnace in an active gas, as for example,hydrogen, and heated to an elevated temperature of about 850 C. By usingan appropriate solder such as a copper-silver eutectic solder, the seals24 are made at this temperature. During the heating, prior to theformation of seals 24, the charged material releases a portion of thehydrogen and, with the hydrogen in the furnace, replaces any otheratmosphere and any sorbed gases removed from the internal elements bythe heating as well as the rather impure hydrogen of the furnaceatmosphere. Since the pressure of hydrogen in the device is that of thefurnace and the temperature is known, a known quantity of hydrogen isenclosed in the device. An appropriate material such as copper-silvereutectic solder is placed at seals 24 and, at the known temperature, theseals are completed, trapping the gas therein. The device is thenallowed to cool slowly and the hydrogen sealed in the envelope isabsorbed by the material of layer 15. Since the quantity of hydrogenwhich must be removed to properly reduce the pressure is known, theamount of material 15 required can be predetermined. Specifically,sufficient material is provided so that, at room temperature, thehydrogen pressure will be less than l torr, preferably in the range oftorr. If it is desired that the equilibrium hydride alloy be an evenbetter getter at room temperature for gaseous im purities releasedduring arcing, a gold-nickel eutectic solder may be used and the sealingtemperature raised to approximately 1000 C.

In one complete operation, the performance of the invention followed thefollowing schedule. The ferrous and refractory parts are outgassed byheating to 1000 C. for /2 hour. The copper electrodes are outgassed at900 C. for /2 hour. The trigger electrode and the reservoir stockingsand particular getter parts 27 and 28 are charged with hydrogen byheating to a temperature of 1000 C. in an atmosphere of pure dryhydrogen and cooling slowly over a 2 hour period. This is repeated 5times to assure complete charging. The stainless steel end assembliesare brazed with a .0005 layer of copper. Ceramic cylinders 3 aremetallized at the end surfaces with a second layer of 0.0005 layer ofcopper over a 0.0001" first layer of molybdenum-manganese alloy (16 wt.percent Mn-remainder molybdenum). After plating the ceramic washers areheated for 10 min. At a temperature of 1000 C. to sinter the copper tothe first layer.

The parts are assembled as shown in the drawing with a 500 g. weight tomaintain sealing pressure and a 0.002 thick brazing ring of 28 wt.percent copper and 72 wt. percent Ag between metal and ceramic part. Theassembly is placed in a gas-tight furnace chamber which is then flushedat room temperature for /2 hour with hydrogen to remove all other gasesfrom the furnace. After flushing, the temperature is slowly raised at arate of approximately 5 /min. to a temperature of 750 C. to cause theTiH parts to evolve very pure hydrogen so as to fill the interior ofenvelope 2. The temperature of the envelope is then raised as rapidly aspossible to 820 C. and held for 2 minutes to cause the metal-toceramicseals to be formed. The furnace is then allowed to cool slowly (about 2hours) to room temperature and the formed device is removed.

It can be seen from the foregoing that the process of the presentinvention accomplishes several objectives. Due to the provision of acharged layer such as titanium hydride in the device prior to theevacuation method, the fabrication of much larger devices having muchlarger volumes to be evacuated is permitted without the necessity ofproviding large quantities of titanium or other active metal. Suchprovision is expensive and may be quite inconvenient, as for example, inthe case of the trigger layer of triggered vacuum discharge deviceswherein an overly thick layer of titanium might interfere with properoperation of the device.

The higher absorption provided by the present invention also allows theincorporation of suflicient hydrogen for long life without causing ahigh partial pressure of hydrogen gas in the device. Furthermore, thismethod enables the evacuation of the device down to the extremely lowpressure required for proper operation and simultaneously achieves finalsealing of the device and loading of the trigger layer with the requiredactive gas. The evacuation is accomplished without the use of any vacuumpumps or-other evacuating apparatus.

The device illustrated also includes end closure member 4 whichillustrates a further feature of the invention. Member 4 includes anannular groove 25, shielded from the primary discharge by an overhangingannulus 26. A quantity of metallic hydride is placed therein, as forexample, in the form of particles 27 enclosed in a wire mesh stocking28. The stocking may, for example, be fabricated of molybdenum. Despitethe use of a previously charged coating as described above, it may beinconvenient to provide suflicient active metal in layer 15 so as toproperly absorb all of the charging gas in envelope 1. The additionalmaterial 27 has the same characteristics as the trigger electrode inthat it also absorbs hydrogen. By appropriate adjustment of the size ofthe envelope and of the quantity of metal, the pressure can be loweredand maintained in the range of 10" torr.

The provision of active metal particles 27 is also of great importancein devices designed for frequent or nearly continuous initiation andtermination of the primary discharge. During such frequent use, thehydrogen in layer 15 may be depleted to the point where it is no longersuflicient to properly initiate the primary discharge. This may be dueto entrapment of hydrogen ions on the shield or other walls and coveringthereof with metallic particles sputtered from the electrodes. At thistime, the device is simply heated to an elevated temperature, such asabove 600 C. in the case of titanium and hydrogen, so that the particles27 releases the gas therein to the atmosphere of the envelope. Uponcooling, the metal of layer 15 absorbs part of the gas and thus isreplenished. After such an operation the percentage content of hydrogenin the trigger electrode will be the same as in particles 27. With themass of the latter much greater than the former, nearly completereplenishment of the trigger electrode may be accomplished many timesduring the useful life of the device.

It is noted that the specific illustration of the cylindrical surface 25is only exemplary, and that any appropriate means of providing anadditional quantity of gas-absorbent metal in a relatively cool portionof the device would be sufiicient. For example, this reservoir may beconveniently located in a readily-heated appendage of the gap device.

For a more detailed description of the operation of triggerable vacuumdischarge devices, reference is again made to my U.S. Patent 3,087,092.In general, a source of triggering potential is applied between wire 18and electrode 6 by connection through end wall assembly 2. The primaryvoltage is applied between end wall assembly 2 and electrode supportmember 11 soas to create a strong electric field between primaryelectrode 5 and 6. Due to the hard vacuum within envelope 1, very highvoltages may be applied between electrodes 5 and 6 without causingbreakdown. When a trigger pulse is applied through wire 18 and gap 17, aspark discharge is initiated across groove 20 and the establishment of atrigger arc causing a heating of the metal film 15 and a consequentdischarge of hydrogen into the vicinity of the are where 7 the hydrogenatoms'are ionized. Magnetic forces then propel the resultant hydrogenplasma into the primary gap, thus enabling initiation of the primarydischarge. The exact time of breakdown of the main gap (which. iscontrollable to a matter of microseconds) is when the plasma is injectedinto the gap.

It is noted that the pretreatment of layer. 15 may be done prior to theapplication thereof to the ceramicmember .14, in which case the titaniumhydride may be painted thereon, fired to the ceramic in a vacuum andheated in hydrogen to compensate for any loss during the firing.Alternatively, the elemental titanium may be adhered to the ceramic andthe resultant assembly may then be cycled in hydrogen to accomplish thepretreatment.

Although this invention is shown and described in connection with twofixed electrodes and a fixed gap, it is noted that the invention isparticularly applicable to devices wherein the normal position is opencircuit and breakdown of the gap is pulsed in accord with the operationof a trigger vacuum gap and the arc initiate-d there-v by may beextinguished by moving one electrode into direct physical contact withthe other electrode, as for example, in a vacuum switch of thecircuit-breaker type or re-closer type or other vacuum switches. As usedherein, the terms vacuum gap device is intended to include all suchdevices whether the electrodes are fixed or movable.

While I have shown and described several embodiments of my invention, itwill be apparent to those skilled in the art that many changes andmodifications may be made without departing from my inventionin itsbroader aspects; and I therefore intend the appended claims to cover allsuch changes and modifications as falliwithin the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An improved triggerable vacuum discharge device including a primaryg-as across which a discharge is initiated by injection of an ionizedactive gas,said device being adapted for the replenishmentof said activegas without breaking the hermetic seal ofsaid envelope, and comprising:a hermetically sealed envelope; a pair of primary electrodes disposed insaid envelope and defining a primary gap therebetween; trigger meansadapted to inject a gaseous plasma into said primary gap to initiate adischarge therebetween, said trigger means comprising an activegasrcharged metal; and an additional quantity of the same activegas-charged metal disposed in said envelope at a location removed fromthe area of said discharge and accompanying heating thereof so as tomaintain said additional quantity of metal below the temperature rangein which a substantial portion of said gas is released therefrom duringarcing, but adapted to be selectively heated to cause the evolution ofactive gas therefrom to recharge said trigger means.

2. An improved'triggerable vacuum discharge device as claimed in claim 1wherein said additional quantity of gas-charged metal comprisesparticle's enclosed in a mesh stocking 3. An improved triggerable vacuumdischarge device as claimed in claim 1 wherein said additional quantityof gas-charged metal is shielded from the discharge.

across said primary gap.

4. An improved triggerable vacuum discharge device as claimed in claim 1wherein said metal is selected from the group consisting of titanium,hafnium, thorium and zirconium and said active gas is hydrogen.

References Cited UNITED STATES PATENTS 3,087,092 4/1963 Latferty 315-3303,188,514 6/1965 Cobine 313197 X JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant'Examiner.

1. AN IMPROVED TRIGGERABLE VACUUM DISCHARGE DEVICE INCLUDING A PRIMARYGAS ACROSS WHICH A DISCHARGE IS INITIATED BY INJECTION OF AN IONIZEDACTIVE GAS, SAID DEVICE BEING ADAPTED FOR THE REPLENISHMENT OF SAIDACTIVE GAS WITHOUT BREAKING THE HERMETIC SEAL OF SAID ENVELOPE, ANDCOMPRISING: A HERMETICALLY SEALED ENVELOPE; A PAIR OF PRIMARY ELECTRODESDISPOSED IN SAID ENVELOPE AND DEFINING A PRIMARY GAP THEREBETWEEN;TRIGGER MEANS ADAPTED TO INJECT A GASEOUS PLASMA INTO SAID PRIMARY GAPTO INITIATE A DISCHARGE THEREBETWEEN, SAID TRIGGER MEANS COMPRISING ANACTIVE GAS-CHARGED METAL; AND AN ADDITIONAL QUANTITY OF THE SAME ACTIVEGAS-CHARGED METAL DISPOSED IN SAID ENVELOPE AT A LOCATION REMOVED FROMTHE AREA OF SAID DISCHARGE AND ACCOMPANYING HEATING THEREOF SO AS TOMAINTAIN SAID ADDITIONAL QUANTITY OF METAL BELOW THE TEMPERATURE RANGEIN WHICH A SUBSTANTIAL PORTION OF SAID GAS IS RELEASED THEREFROM DURINGARCING, BUT ADAPTED TO BE SELECTIVELY HEATED TO CAUSE THE EVOLUTION OFACTIVE GAS THEREFROM TO RECHARGE SAID TRIGGER MEANS.